Re: Integration of SCST in the mainstream Linux kernel

[Vladislav Bolkhovitin <vst@vlnb.net>]

Erez Zilber wrote:
> Bart Van Assche wrote:
> 
>>As you probably know there is a trend in enterprise computing towards
>>networked storage. This is illustrated by the emergence during the
>>past few years of standards like SRP (SCSI RDMA Protocol), iSCSI
>>(Internet SCSI) and iSER (iSCSI Extensions for RDMA). Two different
>>pieces of software are necessary to make networked storage possible:
>>initiator software and target software. As far as I know there exist
>>three different SCSI target implementations for Linux:
>>- The iSCSI Enterprise Target Daemon (IETD,
>>http://iscsitarget.sourceforge.net/);
>>- The Linux SCSI Target Framework (STGT, http://stgt.berlios.de/);
>>- The Generic SCSI Target Middle Level for Linux project (SCST,
>>http://scst.sourceforge.net/).
>>Since I was wondering which SCSI target software would be best suited
>>for an InfiniBand network, I started evaluating the STGT and SCST SCSI
>>target implementations. Apparently the performance difference between
>>STGT and SCST is small on 100 Mbit/s and 1 Gbit/s Ethernet networks,
>>but the SCST target software outperforms the STGT software on an
>>InfiniBand network. See also the following thread for the details:
>>http://sourceforge.net/mailarchive/forum.php?thread_name=e2e108260801170127w2937b2afg9bef324efa945e43%40mail.gmail.com&forum_name=scst-devel.
>>
>>  
> 
> Sorry for the late response (but better late than never).
> 
> One may claim that STGT should have lower performance than SCST because
> its data path is from userspace. However, your results show that for
> non-IB transports, they both show the same numbers. Furthermore, with IB
> there shouldn't be any additional difference between the 2 targets
> because data transfer from userspace is as efficient as data transfer
> from kernel space.

And now consider if one target has zero-copy cached I/O. How much that 
will improve its performance?

> The only explanation that I see is that fine tuning for iSCSI & iSER is
> required. As was already mentioned in this thread, with SDR you can get
> ~900 MB/sec with iSER (on STGT).
> 
> Erez